The present invention relates to liquid crystal display panel substrates having a member having anisotropic absorption, and liquid crystal display devices such as liquid crystal display panels using the substrates.
The range of use of liquid crystal displays is increasing because they are characterized, for example, in that they have a high display quality, are thin and consume only low electric power. In recent years, there is a growing demand for a good color reproducibility and a high contrast ratio with an increase of the range of use of the liquid crystal displays as mobile monitors (e.g. monitors for mobile phone and monitors for digital still camera), monitors for desktop personal computers, monitors for printing and design, medical monitors and liquid crystal televisions. Particularly in the case of the liquid crystal televisions, the presentation of a black color is considered very important and a high luminance is urgently required.
A taste for tone has a great influence on the image quality of the liquid crystal televisions. For example, in Japan, white display in the liquid crystal televisions is not set at achromatic display in chromatics but set so as to attain a high color temperature of 9300K or above 10000K in some cases.
In a liquid crystal display device using a pair of polarizing plates, white display and black display are markedly affected by the transmission characteristics of a perpendicular polarizing plate and a parallel polarizing plate as the polarizing plates used.
That is, a black color is affected by the perpendicular transmittance of the polarizing plates and a white color is affected by their parallel transmittance. A low perpendicular transmittance and a high parallel transmittance are required for attaining a high contrast ratio. In the case of polarizing plates obtained by incorporating iodine into a stretched poly(vinyl alcohol) resin, the contrast ratio in the shortwave region is often low. The reason is considered as follows: it is difficult to control the order parameters of the resin and iodine completely.
Therefore, transmitted light in the shortwave region, i.e., blue transmitted light is higher in black display and lower in white display as compared with transmitted light in the long-wave region. When white display is set so as to attain a high color temperature, namely, when it is set at a highly bluish white color, blueness is intensified in black display. This intensification is disadvantageous in the liquid crystal televisions in which the presentation of a black color is considered important.
As a means for solving the problem of the tone difference between black and white due to the above-mentioned polarizing plates, a tone correction polarizing-plate technique has been reported in the non-patent document 1 described below.
In addition, the correction of tones with low gradation in a liquid crystal display device in PVA mode is described in the patent document 1 described below.
Usually, display in a liquid crystal display device is carried out by applying an electric field to liquid crystal molecules in a liquid crystal layer held between a pair of substrates to change the direction of orientation of the liquid crystal molecules, and utilizing a change in the optical characteristics of the liquid crystal layer caused by the change of the direction of orientation. In so-called active-drive type liquid crystal display devices having a switching element (e.g. a thin-film transistor) for each pixel, an electrode has heretofore been provided on each of a pair of substrates holding a liquid crystal layer between them, so that the direction of an electric field to be applied to the liquid crystal layer may be substantially perpendicular to the boundary surface between the substrate and the liquid crystal layer. A display method adopted in the active-drive type liquid crystal display devices is represented by a twisted nematic (TN) display method in which display is carried out by utilizing the optical rotary power of liquid crystal molecules constituting the liquid crystal layer. In the liquid crystal display devices adopting this TN method, the narrow angle of field of view is considered the greatest problem.
On the other hand, IPS method has been developed in which an electric field generated by the use of an inter digital electrode formed on one of a pair of substrates is adjusted so as to have a component substantially parallel to the surface of the substrate having the inter digital electrode formed thereon, and liquid crystal molecules constituting a liquid crystal layer are rotated in a plane substantially parallel to the substrate, whereby display is carried out by utilizing the birefringence of the liquid crystal layer. This IPS method has advantages such as a wider angle of field of view and a lower load-carrying capacity because of the in-plane switching of the liquid crystal molecules as compared with the conventional TN method. Liquid crystal display devices adopting the IPS method are considered hopeful as novel liquid crystal display devices substituted for those adopting the TN method and have been in rapid progress in recent years. In addition, an IPS method has been developed in which the transmittance is improved by forming at least one of electrodes for applying an electric field to a liquid crystal layer, by the use of a transparent electroconductive film.
Such liquid crystal display devices adopting the IPS method (hereinafter referred to as “IPS-TFT-LCD”), which are excellent in visual angle characteristics (luminance contrast ratio, and gradation•tone inversion) and give a light display, are very useful in monitors and televisions, which have a large display region. On a pair of substrates holding a liquid crystal layer between them in the liquid crystal display device, an alignment film having an ability to control the orientation of liquid crystal molecules is formed on the boundary surface between each substrate and the liquid crystal layer. However, the development of novel structure and process for a large-sized display device (a large panel) is required for the practical application of IPS-TFT-LCD suitable for a larger screen of 20 inches or more.
Particularly in IPS-TFT-LCD having many level difference structures on the surfaces facing the liquid crystal layer, it is difficult to subject the alignment films to uniform alignment treatment all over a large screen. A margin in the case of subjecting the alignment films to the alignment treatment is remarkably narrower as compared with the conventional TN methods, in particular, a normally-open type TN method (light display at a low voltage and dark display at a high voltage) which is a leading method at present. The reason for the narrow margin is explained in the following items (1) to (3).
(1) As to the level difference structures: In IPS-TFT-LCD, a large number of long and narrow electrodes (referred to as inter digital electrodes in some cases) having a width of about several micrometers have to be provided in principle. Therefore, fine level difference structures are formed. The degree of the level difference depends on the thickness of the electrodes and the shapes of various films formed on the electrodes and is usually 10 nm or more. In a high-transmittance pixel structure, an inorganic insulating film is formed thick and level difference concavities and convexities having a thickness smaller than that of the inorganic insulating film are flattened to a certain degree. Accordingly, the level differences of the alignment film of the high-transmittance pixel structure are attributable mainly to electrodes on an organic insulating film. On the highest film among such films, an alignment film (referred to also as an alignment layer) made of a polymer film of polyimide or the like is formed.
In a conventional mass production technique, an ability to orient liquid crystal molecules (initial orientation) is imparted to the alignment film by subjecting the film to rubbing treatment. On the other hand, cloth for the rubbing is composed of a bundle of fine fibers with a thickness of approximately 10 to 30 μm. Substantially, each of the fine fibers gives a shearing force in a definite direction to the local portion of the alignment layer, whereby treatment for imparting the ability to orient liquid crystal molecules is carried out. As the fibers, there are very fine fibers of several microns, but cloth made of such very fine fibers has not been put to practical use because the cloth for rubbing is required to have stiffness for applying a certain degree of frictional force.
The distance between electrodes in the IPS method is also approximately 10 to 30 μm which is the same as the diameter of the above-mentioned fibers. Therefore, rubbing near the level difference is insufficient, so that the orientation tends to be disturbed. This disturbance of the orientation causes a rise of the black level and a lowering of the image quality (e.g. a decrease in contrast ratio and the nonuniformity of luminance) due to the rise.
(2) As to the angle of orientation: In IPS-TFT-LCD, the direction of initial orientation has to be set in principle so as to be different by a definite angle or more from a direction in which electrodes extend or a direction perpendicular thereto. Here, the electrodes refer to signal wiring electrodes, common electrodes in a pixel and pixel electrodes. In order to determine the direction of initial orientation by rubbing, rubbing with fibers of approximately 10 to 30 μm in a direction at a predetermined angle is necessary as described above. However, owing to the level difference between wirings extending in definite directions (e.g. the signal wiring electrodes, common electrodes in a pixel and pixel electrodes) and their ends, the fibers are drawn in the direction of the level difference from a preset angle, so that the orientation is disturbed, resulting in a lowering of the image quality, such as a rise of the black level.
(3) As to the sinking of a dark level: One of the characteristics of IPS-TFT-LCD is satisfactory sinking of a dark level (a black display). Therefore, the disturbance of orientation tends to be conspicuous as compared with other methods. In the conventional normally-open type TN method, the dark level is obtained when a high voltage is applied. In this case, at the high voltage, most liquid crystal molecules are aligned in the direction of an electric field which is a direction perpendicular to the surface of a substrate. Owing to the relationship between the alignment of the liquid crystal molecules and the location of a polarizing plate, the dark level is obtained. Accordingly, in principle, the uniformity of the dark level is not highly dependent on an initial orientation state at a low voltage. In addition, human eyes are sensitive to the variation of the dark level because they recognize the nonuniformity of luminance as the relative ratio between luminance values and response like a logarithmic scale. Also from this point of view, the conventional normally-open type TN method, in which liquid crystal molecules are forcedly aligned in one direction at a high voltage, is advantageous because of its insensitiveness to the initial orientation state.
On the other hand, the IPS method is sensitive to the disturbance of an initial orientation state because in this method, dark level display is carried out at low or zero voltage. Polarized light introduced into a liquid crystal layer is propagated with almost no disturbance of linear polarization particularly in the case of the following arrangement: a homogeneous alignment is employed in which the directions of orientation of liquid crystal molecules on upper and lower substrates, respectively, are parallel; the axis of light transmission of one of two polarizing plates is parallel to the direction of orientation of liquid crystal molecules; and that of the other is perpendicular to the direction of orientation (this arrangement is called a birefringence mode). This fact is effective in sinking the dark level. Transmittance T in the case of the birefringence mode can be generally expressed by the following equation:T=To·sin2 {2θ(E)}·sin2 {(Π·deff·Δn)/λ}
Here, To is a coefficient and is a numerical value determined mainly by the transmittance of the polarizing plate used in a liquid crystal panel; θ(E) is an angle between the direction of orientation of liquid crystal molecules (the effective optical axis of the liquid crystal layer) and the axis of transmission of polarized light; E is the intensity of an electric field applied; deff is the effective thickness of the liquid crystal layer; Δn is the refractive index anisotropy of the liquid crystals; and λ is the wavelength of light. In addition, here, the product of the effective thickness deff of the liquid crystal layer and the refractive index anisotropy Δn of the liquid crystals, namely, deff·Δn is referred to as retardation. The thickness deff of the liquid crystal layer in this case corresponds not to the thickness of the whole liquid crystal layer but to the thickness of a portion of the liquid crystal layer which is actually changed in the direction of orientation when a voltage is applied thereto. This is because liquid crystal molecules near the boundary surface of the liquid crystal layer are not changed in the direction of orientation owing to the influence of anchoring on the boundary surface even when a voltage is applied thereto. Therefore, when the thickness of the whole liquid crystal layer held between substrates is taken as dLC, there is always the relationship deff<dLC between the thickness dLC and deff and the difference between them can be estimated at approximately 20 nm to 40 nm though it varies depending on the kind of a liquid crystal material used in the liquid crystal panel and the kind of a boundary surface in contact with the liquid crystal layer, for example, a material for alignment film.
As is clear from the above equation, the term sin2 {2θ(E)} is dependent on the intensity of electric field, and the luminance can be adjusted by varying the angle θ, depending on the intensity of electric field E. In the case of normally-close type, a polarizing plate is set so that the angle θ may become zero when no voltage is applied. Therefore, such a method becomes sensitive to the disturbance of the direction of initial orientation.
Thus, in the IPS method, the uniformity of orientation is a very important factor and problems in the rubbing method adopted at present are becoming clear. In general, rubbing alignment treatment involves many problems concerning a rubbing treatment method, such as TFT damage caused by static electricity generated by friction, unsatisfactory display due to the disturbance of orientation caused by the disarray of the ends of fibers of rubbing cloth and dust, and frequent replacement of the rubbing cloth. In order to solve these problems in the rubbing alignment treatment, a so-called “rubbingless alignment method” is investigated and various methods have been proposed. As such a method, a photo-alignment method has been proposed in which liquid crystal molecules are oriented without rubbing treatment by irradiating the surface of a polymer film with polarized ultraviolet light or the like.
As an example of the photo-alignment method, the method disclosed in the non-patent document 2 described below is characterized by orienting liquid crystal molecules in a definite direction by irradiation with polarized light without requiring conventional rubbing treatment. This photo-alignment method is free from problems in the rubbing method, such as scratches on the film surface and static electricity and is advantageous in that it is simpler as a production process when industrial production is taken into consideration. Therefore, it is noted as a novel method for liquid crystal molecular alignment treatment which uses no rubbing treatment.
As to a material for a liquid crystal molecular alignment layer, employment of a high-molecular weight compound obtained by introducing a photoreactive group into the side chain of a polymer has been proposed in order to attain photochemical sensitivity to polarized light. A typical example of the high-molecular weight compound is a poly(vinyl cinnamate). In this case, it is considered that the poly(vinyl cinnamate) exhibits anisotropy in a polymer film owing to dimerization in the side chain portion by light irradiation to orient liquid crystal molecules. In addition, it has been reported that liquid crystal molecules are oriented by irradiating a specified polyimide film with polarized ultraviolet light or the like. In this case, it is considered that the liquid crystal molecular orientation is caused by the decomposition of the polyimide main chain in a definite direction by the light irradiation.
Moreover, it has been proposed that liquid crystal molecules can be oriented in a definite direction by dispersing a low-molecular weight dichromatic azo dye in a polymeric material and irradiating the surface of such a film with polarized light. As to this technique, there is the following patent document 2.
Patent document 1: JP-A-2003-29724
Patent document 2: JP-A-11-133431
Non-patent document 1: SID 03 p. 824-827
Non-patent document 2: W. M. Gibbons et al., Nature, 351, 49 (1991)
As described above, as to the display characteristics of a liquid crystal display device using polarizing plates, the tones of a black display and a white display are remarkably influenced mainly by the difference in spectral characteristics between the perpendicular transmittance and parallel transmittance of the polarizing plates. In particular, the black display is remarkably influenced by the spectral characteristics of the polarizing plates when the polarizing plates are perpendicular to each other. Iodine stretching type polarizing plates commonly used at present are disadvantageous in that they cannot shut out light at about 400 nm completely when they are perpendicular to each other, so that bluishness is shown.
The above-mentioned patent document 1 discloses a technique in which tones are corrected by controlling three pixels of RGB independently. However, in order to achromatize blue transmitted light, green and red transmitted lights have to be increased. When such a method is adopted in black display, the luminance of the black display is increased, so that the contrast ratio is unavoidably decreased. Therefore, the induction of an increase in the luminance of the black display and a decrease in the contrast ratio are not allowable in a liquid crystal television in which the presentation of a black color is considered important. In addition, displaying a black color at different orientation states of crystal liquid molecules in the pixels, respectively, of RGB becomes a cause for the deterioration of characteristics concerning the angle of field of view. Therefore, the above-mentioned technique is not desirable also in this point.
In the above-mentioned non-patent document 1, a dye showing dichromatism in a shortwave range is located outside each of a pair of polarizing plates to make the perpendicular transmittance characteristics of the polarizing plates into those corresponding to achromatism. However, in the tone correction polarizing plate for the achromatization, four polarizing layers are formed and hence processes for adjusting the axes of the individual layers are necessary, so that the load on the production process is unavoidably increased. Moreover, it is also disadvantageous from the viewpoint of productivity that the nonuniformity of the degree of polarization of the polarizing plates results in a nonuniform display quality.
In the above-mentioned tone correction polarizing-plate technique, since there is the influence of the abolishment of polarization by the lowering of the degree of orientation of the dye, the dye cannot be located inside the polarizing plates, namely, the dye cannot be located on a substrate.
In general, a liquid crystal display device is based on the following principle: the polarized state of linear polarized light transmitted by a polarizing plate on the incident light side is changed by a change in the direction of molecular orientation of a liquid crystal layer to control the amount of light transmitted by a polarizing plate on the emerging light side, whereby display is carried out. Ideally, the polarized state is not changed by the liquid crystal layer at all at the time of black display and light from a light source is shut out by the polarizing plate perpendicularly located on the emerging light side.
Therefore, the black display becomes the product of the perpendicular spectral transmittance of the polarizing plate used and the spectral transmittance of a color filter. In detail, although absorption by the substrates, an insulating layer and a transparent electrode occurs, absorption by the polarizing plates and the color filter is substantially dominant. An intermediate tone and a white color obtained by the transmission of light are displayed by the transmission of birefringent light generated by the liquid crystal layer, by the polarizing plate on the emerging light side. Therefore, the white color display is substantially dominantly affected by the parallel spectral transmittance of the polarizing plate used, the birefringent light of the liquid crystals and the spectral transmittance of the color filter.
However, since the degree of polarization of a polarizing plate obtained by incorporating iodine into a stretched poly(vinyl alcohol) resin is lowered in the shortwave region, blue coloration is caused in a black display and transmission for blue is lowered in white display. On the other hand, in a black display, the appearance of leaked light due to light scattering caused by pigment particles forming the color filter layer and the liquid crystal layer is also a cause for a luminance increase and a tone change from the luminance and tone of an ideal black display.
In the above-mentioned patent document 2, a pre-tilt angle is tried to attain by forming a poly(vinyl alcohol) thin film containing an azo dye as an alignment film and aligning molecules of the azo dye slantingly.
However, in the case of the IPS method, the pre-tilt angle of liquid crystal molecules induces asymmetry with respect to the dependence of contrast ratio and chromaticity change on the angle of field of view. Therefore, the absence of the pre-tilt angle is preferable. In the case of irradiation of the poly(vinyl alcohol) thin film containing the azo dye with polarized light, a force for orienting liquid crystal molecules in a desired direction, i.e., orientation-regulating force (referred to also as anchoring strength) is insufficient. Moreover, a considerable irradiation dose is necessary in order to attain a sufficient orientation-regulating force by an existing photo-alignment technique, and this technique requires a longer working process time than does the rubbing method. Therefore, this technique involves a serious problem also from the viewpoint of productivity.
The present invention permits not only reduction of the chromaticity difference between white display and black display, i.e., the above-mentioned problem but also improvement of the contrast by the reduction of the luminance of a black display, by introducing a layer having anisotropic absorption besides the polarizing plates of a liquid crystal display device. Furthermore, the present invention is effective in compensating for the lowering of the degree of polarization of the polarizing plates and hence aims also at increasing the production margin for the nonuniformity of the degree of polarization of the polarizing plates in addition to the improvement of the image quality.